As various semiconductor devices have been developed, their characteristics have evolved as well. For example, a logic circuit or a central processing unit (CPU) should have a dynamic random access memory (DRAM) and a static random access memory (SRAM) merged together.
When these memory devices are merged, gate oxide layers with different thicknesses must be formed on one chip so as to preserve the respective characteristics of the devices. Also, even when these devices are used without being merged together, gate oxide layers with different thicknesses must still be formed on one chip to enable operation at different operating voltages.
Such gate oxide layers are thermal oxide layers. Such layers have become thinner in accordance with the development of the design rules. However, when a gate insulating layer is formed by implanting boron as a P-type impurity into a SiO2 substrate so as to form a P-type gate structure, boron penetration occurs due to the thinness of the gate oxide layer. The boron penetration degrades the characteristics of the thin film transistors (TFTs) by varying the threshold voltage.
In order to prevent the boron implanted into the SiO2 substrate from penetrating a channel region, an oxynitride layer (also called a nitrided oxide layer) is provided. The oxynitride layer may be formed by a growing method performed in a furnace under a NO, N2O, or NH3 gas atmosphere, or by a plasma enhanced chemical vapor deposition (PECVD) method.
The PECVD method is disadvantageous in that it increases production costs due to the need to purchase PECVD equipment. The growing method performed in a furnace is also disadvantageous in that it increases the process/fabrication time, because an oxide layer is formed and then an oxynitride layer is formed by thermally treating the oxide layer under a nitrogen gas atmosphere. Further, the method for growing the oxynitride layer in a furnace is disadvantageous in that it further increases production cost, since an apparatus for implanting nitrogen gas, an apparatus for neutralizing harmful gas, and a plurality of stabilization apparatus are required to grow the oxynitride layer in a furnace.
To clarify multiple layers and regions, the thickness of the layers are enlarged in the drawings. Wherever possible, the same reference numbers will be used throughout the drawing(s) and accompanying written description to refer to the same or like parts. As used in this patent, stating that any part (e.g., a layer, film, area, or plate) is in any way positioned on (e.g., positioned on, located on, disposed on, or formed on, etc.) another part, means that the referenced part is either in contact with the other part, or that the referenced part is above the other part with one or more intermediate part(s) located therebetween. Stating that any part is in contact with another part means that there is no intermediate part between the two parts.